1. Field of the Invention
The present invention relates to a metal oxide electrochemical psedocapacitor employing metal oxide as an electrode, and more particularly, to a metal oxide electrochemical psedocapacitor employing an organic electrolyte.
2. Description of the Related Art
Much attention is paid onto a metal oxide electrochemical psedocapacitor as a new energy storage system, because this system retains a large energy storage capacitance when is compared with the conventional electrolytic capacitors. Metal oxides such as nickel oxide (NiO), ruthenium oxide (RuO2), cobalt oxide (Co3O4), manganese dioxide (MnO2), etc. and electrolytes such as aqueous hydrosulfuric acid, aqueous kalium hydroxide, etc. are utilized for the conventional metal oxide electrochemical psedocapacitor.
The metal oxide electrochemical psedocapacitor possesses many common technical parts included in a lithium secondary battery and EDLC (electric double layer capacitor). For example, most of the metal oxides and the electrolytes utilized for the metal oxide electrochemical psedocapacitor have been utilized as a cathode material for the lithium secondary battery. However, the metal oxide electrochemical psedocapacitor shows different electrochemical characteristics with those of the lithium secondary battery. For example, they have different cyclic voltammograms. In the cyclic voltammogram, the battery shows a large peak, however, the capacitor including the psedocapacitor shows a quadrilateral shape. In addition, in a charging/discharging behavior, the battery shows a plateau portion, however, the capacitor shows a decreasing straight line of a voltage along with time.
When compared with the EDLC, the psedocapacitor shows the same electrochemical behavior of a common capacitor. However, EDLC employs an activated carbon as an electrode, while the metal oxide electrochemical psedocapacitor employs a metal oxide as the electrode.
An aqueous electrolyte has been used for the metal oxide electrochemical psedocapacitor. However, the aqueous electrolyte has a limitation of exhibiting an electrochemically stable region of 1.0V or less. Accordingly, the energy amount stored by the capacitor is disadvantageous. An energy storage capacitance can be represented by the following equation of (1).
E=xc2xd CV2xe2x80x83xe2x80x83(1)
In the equation (1), E represents the possible amount of the electric energy, C represents a capacitance and V represents an operating voltage of the capacitor.
According to the equation (1), the possible amount of the electric energy which can be stored in the capacitor is proportional to the square times of the voltage of the capacitor. When the aqueous electrolyte is used for the capacitor, the maximum voltage is 1.0V. Therefore, E=xc2xdC. However, when an organic electrolyte is used for the capacitor, the operational voltage becomes 2.3V or more. Therefore, E=xc2xd Cxc3x972.322 and E=xc2xd Cxc3x975.3. This means that the amount of the electric energy can be increased by 5.3 times or more according to the electrolyte. The EDLC having the same objective with that of the metal oxide electrochemical psedocapacitor utilizes both the aqueous and organic electrolyte.
However, the metal oxide electrochemical psedocapacitor utilizes only the aqueous electrolyte because this system stores the electric energy in a different manner with that of the EDLC. The EDLC utilizes a physical separation phenomenon of a charge by an electrical double layer formed at an interface of an electrode and the electrolyte. Accordingly, the velocity of charging and discharging is very fast and receives not much effect by the electric conductivity of the electrolyte. However, the metal oxide psedocapacitor utilizes an electrochemical faradaic reaction at an electrode as an energy storage system and proton included in the electrolyte is regarded as a working ion. Accordingly, the organic electrolyte containing a minute amount of the proton is regarded as not exhibiting a good performance in the psedocapacitor.
As for an electrochemical energy storage system utilizing an organic solution and salt as an electrolyte, various methods has been reported by journals and patents for the lithium secondary battery and EDLC. However, the same methods applied for the lithium secondary battery and the EDLC cannot be applied for the metal oxide electrochemical psedocapacitor which is a different electric energy storage system from them. The following two researches seem have a little concern with a metal oxide electrochemical psedocapacitor utilizing the organic electrolyte.
First, S. Passerini, J. J. Ressler, D. B. Le, B. B. Owens and W. H. Smyrl, xe2x80x9cV2O5 Arogel-conducting substrate composites characterization and use as supercapacitor electrodesxe2x80x9d, in the proceedings of the symposium on electrochemical capacitors, F. M. Delnick and M. Tomkiewicz, Editors, The Electrochemical Society Proceedings Series PV 95-25, P. 86(1995), can be illustrated. According to this report, PC is used as an organic solvent and LiClO4 is used as a salt for the organic electrolyte. Also, vanadium oxide (V2O5) aerogel is used as an electrode material. The performance of the electrode disclosed in this report shows a different performance with that of the capacitor. For example, a graph of CV does not show an ideal quadrilateral shape of a capacitor.
Next, Kuo-Chuan Liu and M. A. Anderson, xe2x80x9cThe effects of electrolytes on nickel oxide-based electrochemical capacitorsxe2x80x9d, in the proceedings of the symposium on electrochemical capacitors II, F. M. Delnick, D. Ingersoll, X. Andriue and K. Naoi, Editors, The electrochemical society proceedings series PV 96-25, P. 97(1996), can be illustrated. In this report, LiClO4-PC system is used as for the organic electrolyte as in the previous report and nickel oxide (NiO) is used as for the electrode material.
Accordingly, it is an object in the present invention to provide a metal oxide electrochemical psedocapacitor which can employ an aluminum current collector and has a wide and stable electrochemical region and an increased amount of electric energy by using an organic electrolyte instead of an aqueous electrolyte which has been used for the conventional metal oxide electrochemical psedocapacitor having a narrow and stable electrochemical region.
Another object of the present invention is to provide an electric energy storage system having an improved electric conductivity by employing an organic electrolyte including both a lithium salt and an ammonium salt.
To accomplish the object, there is provided in the present invention a metal oxide electrochemical psedocapacitor comprising a plurality of electrodes, an organic electrolyte including a solvent and a solute and a separator inserted between the electrodes for preventing a contact between the electrodes.
The preferred solvent is PC (propylene carbonate) and AcN (acetonitrile) and a preferred solute is a lithium salt and an ammonium salt. A preferred mixing ratio of the lithium salt and the ammonium salt is in a range of 4:6-6:4 in a molar ratio.
As for the lithium salt, LiBF4 (lithium tetrafluoroborate), LiClO4 (lithium perchlorate), LiPF6 (lithium hexafluorophosphate), etc. can be advantageously utilized and as for the ammonium salt, Et4NBF4 (tetraethylammonium tetrafluoroborate), Et4NPF6 (tetraethylammonium hexafluorophophate), Et4NClO4 (tetraethylammonium perchlorate), MeEt3NBF4 (triethylmethylammonium tetrafluoroborate), etc. can be advantageously utilized.
The other object of the present invention can be accomplished by an electric energy storage system comprising a plurality of electrodes, an organic electrolyte including an organic solvent, a lithium salt and an ammonium salt and a separator inserted between the electrodes for preventing a contact between the electrodes.